1. Field of the Invention
The invention is directed to a method for sealing membrane modules, particularly those in which a bundle of tubes or a bundle of hollow fibers with a plurality of tubes or hollow fibers, particularly porous tubes or hollow fibers, having a membrane are held at the end thereof by a head plate and is tightly sealed. Tubes and tube bundles, particularly hollow fiber bundles, which are made of a plastic or a plastics composite, possibly also with a layered construction, a ceramic, or a metal are used for this purpose.
2. Related Art
In fuel cells, a fuel component is conveyed in bundled tubes or hollow fibers of this kind. For this reason, a tube of this kind, which is designed as a microreactor, is also routinely provided with a membrane through which the fuel components reacting with one another are separated.
Another area in which tube bundles of this kind are used is in the refining of propellants made of biologically renewable materials, e.g., bioethanol. When bioethanol flows through a tube forming a semi-permeable membrane, the water present in the bioethanol is extracted through the membrane. Membrane modules of this type are described, for example, in WO 2006/087214.
Tube bundles of the kind mentioned above are held at their ends in a head plate and sealed by potting. Accordingly, the ends of the tube bundles are enclosed with one another and, at the same time, a tight sealing is carried out relative to a housing, a pipeline, or the like using sealing rings. Synthetic resins, e.g., epoxy resin, or, as is described in WO 2007/076855, metals and metal alloys are used for potting.
The use of different materials which, of course, have different thermal expansion coefficients leads to substantial thermal stresses which can cause cracks, fractures, and so on, and which can therefore give rise to leaks in the region of the head plate, which routinely leads to a breakdown of the entire apparatus.
Besides frequent leaks due to defective or poorly fitting seal rings, fractures of the head plates also occur routinely. In this connection, two typical forms of fracture can be distinguished. One form of fracture are circular, crescent-shaped fractures that extend axially in an outer layer of the head plate.
Radial fractures are inconspicuous upon visual inspection of the potting surface. However, these cracks are clearly discernable when a mounting ring holding the head plate is removed. These cracks are apparent under the sealing rings between the mounting ring and the head plate and propagate in the crystalline portions of the head plate, often branching out, and it is quite possible for the entire piece of head plate to break off.
To detect defects of this kind, which can routinely result in leaks, it is necessary to detach the mounting ring, remove the head plate with tube bundles and a housing and subject them to a visual inspection. If defects are discovered, these inserts of the membrane module are simply replaced. This is cumbersome and expensive.